1. Field of the Invention
The present invention relates to encoding of picture signals, and more particularly to the so-called efficient picture encoding.
2. Description of Background Information
The predictive encoding such as DPCM, quasi-instantaneous companding DPCM, and ADPCM is known as the so-called efficient picture encoding in which the average bit number per picture element, or sampled value, is reduced for the purpose of data compression.
In general, there is a correlation of a large degree between adjacent picture elements in the case of ordinary pictures. For instance, when the sampling of a video signal is performed, there will be only a small difference between successive two of a plurality of sampled values obtained consecutively. The predictive encoding scheme utilizes this statistical characteristic, in such a manner that a prediction error is generated between the current value of a picture element and a predicted value obtained by predicting a current value of the picture element using a former value of the picture element, and the prediction error is converted to code data.
FIG. 1 is a block diagram showing a DPCM encoder. As shown, picture element data x.sub.n corresponds to the sampled value which is consecutively obtained by the sampling of a video signal. The picture element data x.sub.n is supplied to a subtracting circuit 1 in which output data of a prediction circuit 5 is subtracted from the input data, i.e., the picture element data x.sub.n. The output data of the subtracting circuit 1 constitutes the prediction error, and is supplied to a quantizing circuit 2. The quantizing circuit 2 is constructed to generate a code representing a quantization representation value obtained by quantizing the prediction error. The output of the quantizing circuit 2 is supplied to an output terminal OUT and to a representation value generating circuit 3. The representation value generating circuit 3 is constructed to generate data representing a quantization representation value (referred to as representation value hereinafter) corresponding to the supplied code data. The output of the representation value generating circuit 3 is supplied to a summing circuit 4 in which the input data is added to output data of a prediction circuit 5. The output of the summing circuit 4 is supplied to the prediction circuit 5 as a decoded value. The prediction circuit 5 is constructed to perform, for example, a one-dimensional prediction, and it supplies the output of the summing circuit 4 delayed by one sampling period, to the subtracting circuit 1 and the summing circuit 4, as a prediction value of the current picture element data x.sub.n to be converted to code data.
In the DPCM encoder having the construction described above, the quality of pictures obtained by decoding the coded video signal is greatly affected by the characteristics of the quantizing circuit 2 and the representation value generating circuit 3. For instance, if the minimum representation value of the quantizing circuit 2 is large, which in turn increases the so called granular noise which vibrate in the same variation width as the minimum representation value. If, on the other hand, the maximum representation value of the quantizing circuit 2 is small, the so called slope-overload effect is encountered in which the decoded video signal fails to track the original video signal at positions where the signal level changes rapidly.
Because of these reasons, coding schemes such as quasi-instantaneous companding DPCM, ADPCM have been developed, in which the characteristics of the quantizing circuit 2 and the representation value generating circuit 3 are varied in response to the character of image.
In the quasi-instantaneous companding DPCM, picture element data groups are divided into blocks, and the dynamic range of the prediction error in each block is calculated previously so that the characteristics of the quantizing circuit 2 and the representation value generating circuit 3 are controlled for each block by switching operations.
In the ADPCM, on the other hand, the characteristics of the quantizing circuit 2 and the representation value generating circuit 3 are controlled for each picture element by switching operations. More specifically, the ADPCM is a coding scheme such that a circuit, in which a plurality of quantizing circuits having different dynamic ranges are used as the quantizing circuit 2, and the input data is quantized by one of the plurality of quantizing circuits designated by range control data r, is provided, and the range control data r is generated for each picture element in response to the representation value of a picture element which has been converted to code data at the previous time.
In the ADPCM encoder constructed as described above, the quantizer for the picture element of the next point of time is determined only by using the prediction error of the current picture element. Therefore, a problem has been encountered that the most appropriate quantizer is not necessarily selected for each picture element, giving rise to the increase in errors between the coded picture signal and the original picture signal. Furthermore, in the ADPCM when an edge portion (leading edge or trailing edge) of the original picture signal is processed, the dynamic range of the quantizer for the next quantizer increases only after the first picture element in the edge portion is converted to a code word corresponding to a high-order representation value in a quantizer having a low dynamic range. Hence, a delay of tracking to the original picture signal is generated, causing the slope over-load effect.